Bidimensional cationic ordering and thermal dependence in beta-PbxV2O5 bronzes

The monoclinic lead vanadium oxide bronzes beta-PbxV2O5 were investigated f or their cationic ordering within the host lattice. Superstructure reflecti ons appear on X-ray powder patterns beyond x approximate to 0.30. For the f ormal x = 0.30 lead ratio, the crystal structure was refined from X-ray sin gle crystal data in the (a, 2b, c) unit cell, a = 15.478(10) Angstrom, 2b = 7.288(5) Angstrom, c = 10.123(6) Angstrom, beta = 109.29(1)degrees, S. G. P2(1)/m, and final R = 0.047, R-w = 0.054. It shows evidence of an intratun nel ordering involving a pb(2+) zigzag sequence running parallel to the b a xis while ordered and disordered tunnels alternate along the a axis. Pb2+ d isplay different interaction with the V-O framework according to the kind o f tunnels it occupies. Electron diffraction patterns confirmed our X-ray re sults. A supplementary 2a periodicity was sometimes observed in small cryst als as a subcondition of the previous ordering. Weissenberg photographs and electronic diffraction patterns allowed the proposition of a bidimensional cationic ordering model in the fourfold (2a, 2b, c) unit cell. In that cas e, the intertunnel ordering involves strong local Pb2+ electrostatic effect s despite the shielding of the tunnel walls. The thermal analysis (DTA and high temperature electronic microscopy) evidenced the irreversible disappea ring of the superstructure on heating while a second rearrangement is obser ved on cooling. (C) 1999 Academic Press.